Mass spectrometers constitute a class of instruments for analysis of elements, compounds and mixtures. Mass spectrometers are often called "universal detectors" because every known species exhibits a mass spectrum. Mass spectrometry has clear benefits, but is often not effective in identifying components of mixtures nor in providing sufficient information to distinguish among similar materials. The improved mass spectrometer apparatus and method of the invention provides additional information which, used in conjunction with the standard mass spectra, allows one to distinguish spectra of similar materials and makes possible the identification of the components of a sampled mixture without requiring separation of the mixture components prior to mass spectral analysis. Utilizing the apparatus and method of the invention in mass spectrometers can additionally significantly lower detection and analysis costs.
A typical mass spectrometer comprises three sections: an ionizer, a mass filter and an ion detector. The ionizer converts some portion of the analyzed material, usually referred to as the analyte, into gaseous ions. The mass filter disperses the ions according to mass and the ion detector measures the flux or concentration of ions transmitted by the mass filter. A variety of devices exists at present to implement each of the three spectrometer sections. Some discussion of ionizers follows in sufficient detail to explain their operation in accordance with the invention.
Electron impact ionizers are the most commonly used ionizers in mass spectrometers. In them, electrons boil off a metal filament and accelerate to some nominal voltage to form ions by collisions with the analyte. Key ionizer performance characteristics are the nominal electron energy, the energy resolution, i.e., the width of the distribution of electron energies about the nominal value, and the magnitude of the electron beam current. Other ionization devices employ light, e.g., photoionization including multiple photon and multiple color processes; ion beams, e.g., secondary ionization mass spectroscopy (SIMS); high speed atoms, e.g., fast atom bombardment (FAB); a hot filament, e.g., surface ionization; strong electric fields, e.g., field ionization; or radioactive decay.
It is well known to those skilled in the art that variations in ionization conditions can cause major changes in the number and identity of the ions formed from a given analyte. For example, increasing the kinetic energy of electrons in an electron impact ionizer typically increases the total amount of ions produced and also leads to a larger variety of ions formed from molecular precursors. Different types of ions are generated because the ionization process, at higher energies, can also break chemical bonds, thereby forming ionized fragments of the molecular precursor. The distribution of fragment ion intensities can provide useful information that may help identify the analyte, but a plethora of fragment ions may also obscure ions due to the other precursor species in the analyte.
Much of the prior art in mass spectrometry consists of devices for identifying one or more components within a mixture or for providing additional useful information to aid species identification. Some prior art mixture analysis devices cause physical separation of the mixture components before ionization. Examples of such devices are chromatographic devices, such as gas chromatography devices, and devices providing selective desorption from a body, such as disclosed in U.S. Pat. No. 3,548,188, to Nemeth, and the molecular beam time-of-flight device. A different approach to mixture analysis measures the mass distribution of fragment ions generated by intentional decomposition of selected ions found in the mass spectrum of the mixture. Ion decomposition can be effected by collisions with a buffer gas or by the action of light on the ions. Ion fragmentation methods have been described as "taking a mass spectrum of a mass spectrum." Such techniques are often referred to as tandem mass spectrometry or as "MS/MS."
Other prior art devices used for mass spectral analysis of mixtures make use of selective ionization of a target species within a mixture or ionization of a subset of species present including the target species. For example, a prior art device utilizing photoionization via multiple photon absorption, which may also include absorption of light at more than one discrete wavelength for trace vapor detection, is disclosed in U.S. Pat. No. 4,170,736, to Wessel. U.S. Pat. No. 4,433,241, to Boesl, et al., discloses a method and apparatus for determining molecular spectra, and U.S. Pat. No. 4,217,494, to Levy, discloses a method and apparatus for isotope separation. In each of these three patents, the quantity and identity of ions formed is sensitive to relatively small changes in the wavelengths of the photoionization light sources, and the values of the wavelengths used comprise a major part, if not all, of the pertinent experimental conditions.
Some instruments contain a subset of the three components, the ionizer, the mass filter, and the ion detector, that make up a typical mass spectrometer. For example, vacuum ionization gauges, consisting of an electron impact ionizer and a simple cage ion collector, are used to measure gas pressures under high vacuum (10.sup.-3 -10.sup.-8 torr) conditions. Flame ionization detectors for gas chromatography record the instantaneous concentration of ions produced in chromatograph effluent by a high temperature flame. Small, fixed-mass mass spectrometers are used as leak detectors; only helium ions are transmitted to the ion detector.
The present invention has greater generality and wider applicability than the devices and methods of the prior art as represented by the aforementioned photoionization patents. For example, U.S. Pat. No. 4,170,736, to Wessel, discloses a two color photoionization mechanism which is useful for predetermined gaseous molecules and in which the first and second laser wavelengths are both, for at least some portion of the measurement time, at wavelengths corresponding to known energy transitions within the target molecules. Photoionization, in accordance with the present invention, requires no such restrictions on the correspondence between the light wavelengths and any optical properties of the molecules. Non-resonant excitation mechanisms, including direct single photon photoionization, are effective. The invention also is not limited to measurements of predetermined species, even though the preferred embodiment discusses the use of reference spectra of known materials for mixture deconvolution. The invention also applies to measurements of novel, previously unknown species, and provides a mechanism for characterizing such species which generates more useful information than does conventional mass spectroscopy.
The invention also has greater generality than the laser photoionization patent, U.S. Pat. No. 4,433,241, to Boesl, et al. Although this patent discusses generating two-dimensional mass spectra, i.e., ion intensities as a function of both mass and laser wavelength, as with the Wessel patent, it is important to select laser wavelengths coincident with optical transition resonances of the target atoms or molecules. The Levy patent is limited to isotope separation. Levy places restrictions on the allowed wavelengths and wavelength ranges which make his device useful for isotope separation where wavelength specifications are much more restrictive than in practicing the instant invention.
U.S. Pat. No. 3,796,872, to Merren, describes a device for concurrent accumulation of more information than is available from conventional mass spectrometers through the use of plural ion beams, each generated by a different ionizer and each striking a different detector. His ionizers produce a fixed, steady output during the operation of his device.
In embodiments appropriate to cases in which the analyte can be characterized adequately by the response of the total ion signal to time variant ionization, the method and apparatus need not include mass filtration or analysis of the ions.
One object of the present invention is to decrease the cost of analysis of components within a mixture using a spectrometer.
Another object of the present invention is to provide two-dimensional mass spectroscopic information representative of one or more components within an analyte.
One advantage of the present invention is that the components of an analyte need not be separated in order to use mass spectroscopy to analyze one or more of them.
Another advantage of the present invention is that, in accordance therewith, analog or digital signal acquisition and signal processing are usable.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.